Internet-based applications have become increasingly popular in recent years. One class of applications is streaming applications. A user can hereby access multimedia content online with the aid of a streaming protocol. Examples of multimedia content are text, graphics, music, video or a mixture of same. A video clip with music is an example of multimedia content. In this context, online means that immediately after receipt of the first packets at the user, said packets are played back, e.g. at a loudspeaker. A complete downloading, for example of the video clip, is not necessary. In addition to the possibility that a content server provides the multimedia content for retrieval, the multimedia content can be recorded and edited in real time and forwarded directly to a user via a streaming application without buffer storage on a so-called content server. Multimedia content of this kind recorded and transmitted in real time is, for example, a radio program (web radio). Saving the multimedia content, e.g. on a content server, is not necessary in this case.
Furthermore, download applications such as the loading of MP3 pieces of music or MPEG-2 feature films are very popular. In this case, the multimedia content is encoded using a standard compression method according to MP3 or MPEG-2 video for instance, and provided on a content server for downloading. First, the user loads the complete multimedia content onto his device, e.g. onto his computer or mobile phone and after completion of the download process this media content can be played back, e.g. on a monitor.
Furthermore, hybrids of streaming and of the download application are also discussed. For example, with a music distribution service, a user can select a piece of music by using his device, his mobile phone for example, and load it on to his device by payment of a purchase price. In this way, the user is able to listen to the piece of music during the download process and then after it has been loaded on to his device he is able to play back the complete piece of music as often as required. The user hereby receives the impression that the piece of music is streamed, whereas in fact the complete piece of music is nevertheless present on his device after completion of the streaming process. With this hybrid, a point-to-point connection currently exists between the device of the user and the content server.
In addition to this hybrid streaming and download application, point-to-multipoint connections should also be possible. A point-to-multipoint connection is also known as “broadcast” or “multicast” In the following; this hybrid is referred to as a broadcast application. With this application, the same multimedia content is available to several users at the same time. An example of this is a music video that is broadcast live to the user and may be recorded.
In addition to an excellent picture and sound quality, the user of a broadcast application of this kind also expects no transmission errors to occur within the multimedia content. Despite expensive error protection processes, residual errors in the physical layer and radio link layer occur during the transmission of data packets via mobile radio channels. A typical value for a residual error is approximately 1%. Whereas data packets with transmission errors transmitted using a point-to-point connection can be repeated with the aid of an ARQ (Automatic Repeat Request) process, this is generally not economically feasible with broadcast and multicast applications, so that data packets with errors are received. Therefore, with these broadcast applications, the multimedia content is transmitted by a unidirectional transmission channel and a return channel is not available. A broadcast application of this kind is known for example from VHF (radio technology). With the use of the IP protocol (Internet Protocol), data packets containing errors also lead to the complete data packets being lost which means that the multimedia content cannot be played back correctly.
In addition to data transmission containing errors via mobile radio channels, data packets losses can also occur due to a radio cell change. In individual cases, an interruption in data transmission of approximately 10 seconds can occur during a radio cell change. Whilst the data packets for purely broadcast download applications can be reconstructed with the aid of suitable measures such as interleaving, even where there are long interruptions, this method cannot be applied for streaming applications because of the time delay. A number of processes are known for reconstructing data packets that contain errors or have been lost, such as are given in references including:    J. Rosenberg, H. Schulzrinne, “An RTP Payload Format for Generic Forward Error Connection”, IETF RFC 2733, December 1999, http:///www.ietf.org    GPP Doc. GP-031391: “Channel Coding Schemes with Incremental Redundancy for MBMS”, 3GPP TSG=GERAN Meeting #15, Fort Lauderdale, Fla., June 2003    Shokrollahi, “LDPC Codes; An Introduction”, April 2003, http://www.ipm.ac.ir/IPM.homepage/Amin2.pdf    GPP Doc. S4-040230, “Raptor Code Specification for MBMS file download”, 3GPP TSG-SA4 Meeting #31, Montreal, Canada, May 2004, http://www.3gpp.org    Bossert, “Kanalcodierung”, B. G. Teubner, 1992 (“the Error Protection Art”).